1. Field of the Invention
The present invention relates to infrared optical elements for high power infrared lasers, and particularly to low absorption thin-film coatings such as antireflection coatings for CO.sub.2 laser optics.
2. Description of Related Art
Optical elements ("optics") are key components of high power laser systems. Such optics, (e.g. lenses, reflectors, and beamsplitters) focus, reflect, and process the laser beam in a variety of ways, dependent upon the polished curvature of its surfaces and the nature of the thin-film coatings deposited on the polished surfaces of the optics. These thin-film coatings often are responsible for all or a large majority of an optic's absorption of incident optical radiation. Absorbed optical radiation turns into heat, and in order to minimize heating of the optic, thin-film coatings should be designed so that absorption of incident optical radiation is as low as possible.
Carbon dioxide (CO.sub.2) lasers generate high power laser beams and are widely used for industrial purposes such as cutting and welding. These lasers utilize many optical elements including cavity mirrors, beam directing optics and focusing optics. For optical elements that transmit optical radiation, such as a focusing lens or an output coupling mirror ("output coupler"), anti-reflective ("AR") coatings are essential, especially at high power. Without AR coatings a large percentage of the optical radiation would be reflected rather than transmitted. For example, 17% per surface is reflected from zinc selenide (ZnSe), the most widely used transmissive optic material for CO.sub.2 lasers.
AR coatings must be carefully designed and manufactured, otherwise significant optical energy will be absorbed in the coating, which unfortunately generates heat. One significant heat-related problem is the "thermal lens effect" caused by the temperature dependency of the refractive index, the thermal expansion of the optic and to a lesser extent the temperature induced stress dependency of the refractive index. The thermal lens effect can significantly shift the focal length and thereby cause the focal point to shift away from its intended position, a highly undesirable result for many applications such as precision laser-cutting of metal. Moreover, the thermal lens can also introduce aberrations which distort the laser beam and increase the size of the focused spot, which can degrade cutting speed and quality.
One typical low absorption AR coating includes a relatively thick low index fluoride material followed by a relatively thin high index material such as ZnSe deposited over a high index substrate formed of a material such as ZnSe. For the operating wavelength of CO.sub.2 laser optics (10.6 microns), the most common low refractive index fluoride is thorium fluoride (ThF.sub.4). Unfortunately, thorium (Th) is radioactive, and therefore an intensive worldwide search for a suitable replacement has been ongoing for many years, as evidenced by many publications such as Rahe et al., Absorption Calorimetry and Laser Induced Damage Threshold Measurements of AR-coated ZnSe and Metal Mirrors at 10.6 .mu.m., Symposium on Laser-Induced Damage in Optical Materials, SPIE Vol. 1441, p. 113 (1990) The most-investigated replacement candidates are other fluorides, such as BaF.sub.2, YbF.sub.3, PbF.sub.2, YF.sub.3. CeF.sub.3, NaF, PrF.sub.3. Much of the research has concentrated on developing antireflection ("AR") coatings for ZnSe optics, for several reasons: AR coatings are the type of coatings most sensitive to absorption in the fluoride coating material, AR-coated ZnSe lenses are the most common CO.sub.2 laser optic, output couplers are the most critical optic in high power CO.sub.2 lasers, and the performance of industrial CO.sub.2 laser material processing applications are critically dependent on the absorption of the AR-coatings on the ZnSe output coupler and ZnSe lenses.
Only two fluorides are known to yield absorption levels lower than ThF.sub.4, these being PbF.sub.2 and BaF.sub.2. PbF.sub.2 can provide a good single-layer low reflection AR coating due to its higher index, but a second high index layer does not improve the low reflectance of the coating significantly. Without protection from an exterior layer, the absorption of the PbF.sub.2 layer would quickly increase beyond what is typically acceptable for a conventional two-layer ThF.sub.4 -based coating. Another disadvantage of PbF.sub.2 -based coating is that it is difficult to manufacture and process and furthermore, its initial absorption values are believed to be higher than those of a BaF.sub.2 -based AR coating. The refractive index of BaF.sub.2 is such that BaF.sub.2 -based AR coatings have much lower reflectances, including zero theoretical reflectance, equivalent to ThF.sub.4 -based AR coatings. In one study performed by Manfred Rahe for his doctoral dissertation, entitled Untersuchungen zur Herstellung und Charakterisierlng von Hochleisttingsoptiken fur den CO.sub.2 -Laser, on page 117, the absorption of BaF.sub.2 -based coatings was initially observed to be one-half that of a ThF4-based AR coating. However, the low absorption value of the BaF.sub.2 -based coating quickly increased so that in less than 20 days its absorption exceeded that of the ThF.sub.4 -based AR coating, and then continued to increase at a much faster rate. Based on the initial low absorption values observed by Rahe, a suitable BaF.sub.2 -based coating would provide a major advantage in CO.sub.2 laser materials processing applications, providing better beam quality and allowing processing at higher speeds and/or higher laser power levels.
However, as Rahe described, conventional BaF.sub.2 -based coatings exhibit poor "ageing" performance; i.e. in a matter of days the absorption quickly rises to meet, and then exceed that of conventional ThF.sub.4 -based coatings. This unfortunate increase in absorption is attributed to chemisorption of water into the BaF.sub.2 layer. A study reported by Ristau et al., in Round Robin Test on Optical Absorption at 10.6 .mu.m, published in SPIE Vol. 2714, Symposium on Laser-Induced Damage in Optical Materials, 1995, discusses ageing performance in Section 4.3 and, in FIG. 13 presents a graph that illustrates ageing performance. Ristau et al. report that there is a significant ageing effect in typical AR coatings as described above using BaF.sub.2 /ZnSe, which they attribute to adsorption or chemisorption processes of water, or, stated simply, adsorption of water. A much smaller, acceptable increase in adsorption over time was also observed for conventional ThF.sub.4 /ZnSe coatings.
A conventional coating technique to protect a fluoride-based layer such as ThF.sub.4 from water adsorption is to cover it as soon as possible with a second material such as ZnSe, and then depend upon the layer of high index material to prevent the adsorption of ever-present water vapor into the molecular structure of fluoride-based interior layer. An example of such a conventional BaF.sub.2 -based AR coating on ZnSe for 10.6 microns (the CO.sub.2 laser wavelength) is a 11425 angstrom (.ANG.) layer of BaF.sub.2 covered with a 1915 .ANG. layer of ZnSe. An example of a conventional ThF.sub.4 -based coating is similar, with a 10233 .ANG. layer of ThF.sub.4 covered with a 2387 .ANG. layer of ZnSe. In both examples, the outer ZnSe layer has an approximately similar thickness. Because (1) the outer layers of both the ThF.sub.4 - and BaF.sub.2 -based layers have approximately similar thicknesses, and (2) the rapid absorption increase is believed to be caused by water, it has not been clear why the conventional ZnSe layer protected ThF.sub.4 but not BaF.sub.2, although the larger solubility in water of BaF.sub.2 over ThF.sub.4 is thought to be responsible.
In McNally et al., Survey of Available Potential Replacements for Thorilim Fluoride, Annual Tech. Conference Proceedings of the Society of Vacuum Coaters, Vol. 35 pages 169-73 (1992), evaporation fluorides, including BaF.sub.2 were studied. The article concluded that such fluorides would not be good substitutes for ThF.sub.4 in most infrared applications, especially those at 10.6 microns. In summary BaF.sub.2 has not been used effectively as a CO.sub.2 optic thin film coating material, and it would be an advantage to provide an effective and practical low absorption BaF.sub.2 -based thin film AR coating and other coatings such as partial reflector coatings for output couplers, beamsplitters, total reflectors, and various other types of coatings used in high power CO.sub.2 lasers.